net: dsa: mv88e6xxx: use mv88e6xxx_phy_is_internal in mv88e6xxx_port_ppu_updates

Make sure to use existing helper to get internal PHYs count instead of
redoing it manually

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>